The development of wireless communication systems has increased the demand for monolithically integrated, low-cost and low-phase-noise voltage controlled oscillators (VCO:s).
However, due to several factors, it becomes increasingly more difficult to design low phase noise VCO:s as the demand for even higher frequencies (i.e. >20 GHz) increases. One factor is that the so-called Q-factor of resonators decreases with increasing frequency, due to the increase of losses of on-chip varactor diodes and inductors. Another problematic factor is that the switching performance of the active devices (i.e. transistors) in the oscillator circuit deteriorates with increasing frequency. This adds undesirable phase delays and also increases the period during which the transistor generates most noise in the VCO. In addition, in order to increase the ft or fmax of the transistor, the breakdown voltage decreases, this limits the signal swing of the oscillator, thus limiting the phase noise performance.
There is therefore a need for monolithically integrated, low-cost VCO:s with improved phase-noise performance under the above-mentioned circumstances.